Casting readily oxidizable metals



Patented Aug. 9, 1932 UNITED STATES PATENT OFFICE JOHN A. GANN, OFMIDLAND, MICHIGAN, ASSIGNOR TO THE DOW CHEMICAL COMPANY,

OF MIDLAND, MICHIGAN, A CORPORATION OF MICHIGAN CASTING READILYOXIDIZAIBLE METALS No Drawing.

While the present improvements relate more particularly to casting ofmagnesium and so-called light metal alloys, in which magnesium is thepredominant constituent, it will be understood that the improved methodand means involved may also be found adaptable for the casting of othersimilar readily oxidizable metals. Difficulties encountered in meltingand handling, in molten state, magnesium or an alloy largely composed ofthis metal are Wellknown, and those.

difliculties are still present when it is attempted to introduce suchmetal into a mold in any process of casting. For example, when greensand castings are made in the ordinary way by simply pouring the moltenmagnesium or light metal alloy into an untreated sand mold, not only isthere'a pronounced tendency for the metal to burn in the sprue, but thesurface of the casting formed by contact with the complementary surfaceof the mold tends to become oxidized, pitted or both, due to theformation of a powder (presumably an oxide or a nitride of the metal)Where the casting poured in this manner is of relatively heavy sect-ion,it either completely burns up or is badly damaged, due presumably to thetemperature of the metal cast remaining at a relatively hi h point for alonger period of time than is t 6 case with thin-section castings whichchill promptly. Even ifthe impairment of the surface of such castings ascan be made is not serious enough to prevent the cast article from beingfinished .when removed from the mold, the casting has an unsightlyappearance and a certain loss of metal is involved. It is to be noted,of course, that the cleaner the casting, the more easily may thesubsequent finishing operations be carried out.

It has heretofore been proposed to overcome these diificulties,specifically in casting magnesium and alloys containing the same,

by dusting the mold prior to casting the metal I with sulphur orequivalent material capable of giving off in contact with the moltenmetal .a vapor which is more easily oxidizable than magnesium. Thisprocedure, however, has the objection that it is obviously diificult toApplication filed June 80,

1930. Serial No. 465,062.

correctly gauge the amount of sulphur or equivalent material thus usedand particularly if an excess is applied to the surface, either of themold proper or the core, so much vapor may be generated that some of itbecomes entrapped during the period that the metal is solidifying, theresult being blowholes or surface shrinks in the casting.

More recently it has been proposed to avoid the difiiculties encounteredin casting magnesium and its alloys by using a molding compositioncontaining pulverized carbon, pulverized soapstone, magnesium oxide anda light bodied mineral oil. I have found, however, that the use ofmagnesium oxide, which is commercially sold as a fine powder, is notonly unnecessary but detrimental since it makes the mold less porous andthus tends to prevent the ready escape of ases generated during casting.This in itselfi is undesirable since it causes pitting, surface shrinks,and general surface injury to the casting. I have further found that theuse of magnesium oxide in contact with hot magnesium and its alloysaccelerates or' promotes the oxidation of themetal.

Accordingly, among the objects of the present invention is theproduction of an improved molding composition suitable for use incasting readily oxidizable metals such as magnesium and its alloys, theuse of such composition further permitting the roduction of castingshaving a good su ace appearance. Other objects and advantages willappear as the description proceeds.

To the accomplishment of the foregoing and related ends, the invention,then, consists of the method and product hereinafter fully described andparticularly pointed out in the claims, the following description set-00 ting forth in detail means and modes of carrying out the invention,such disclosedmeans and modes illustrating, however, but several of thevarious ways in which the principle of the invention may be used.

The mold which is formed to produce a casting of any desired shape orsize, e. g. a crankcase for an internal combustion engine or likearticle, may be made entirely from my improved molding compositionconsisting of 1 pulverized carbon, a plasticizing agent and anon-aqueous hinder, or may be made from such composition havingincorporated therein cores or metal parts as is well known to the art.

I have discovered that where pulverized or finely divided carbon is usedinstead of sand for forming a mold, that such mold has a much higherheat conductivity than a similar sand mold. This increased heatconductivity causes the hot metal bein poured into the mold to chillpromptly, w ich not only decreases the period wherein rapid oxidationcan take place but produces a fine grain structure casting havingsuperior properties as compared with a like casting made in sand. Inthis connection it is to be noted that carbon is chemically andphysically not an equivalent to sand since it is not an oxide, it iscombustible and has a much higher heat conductivity than sand. I amaware that graphite, powdered coal, and lampblack have been used asfacings and washes for molds used in the casting of iron and steel;however, the problem encountered there is in no way comparable with thatencountered in the casting of an extremely readily oxidizable metal,such as magnesium and its alloys, which has to be handled and poured ata temperature wherein it readily ignites and burns if exposed to theair.

Where using pulverized carbon and oil alone for producing my improvedmolding composition, I have found that the percentage range by weight(of all ingredients) of the carbon lies between 7 5-90 per cent. whilethe oil may vary from 25-10 per cent. The factors varying the percentagedepend principally upon the sieve analysis of the carbon, the'type ofoil, and the nature of the casting to be made. While I have found thatthe molds made from mixtures of pulverized carbon and oil will producegood castings, I normally prefer to add thereto a plasticizing agent (1to 30 per cent) such as soapstone, or some other suitable silicate, suchas bentonite.

Example 1 Where using pulverized carbon and oil alone I have found thatgood castings can be madeby using the following composition 2-- 18%medium body mineral oil mixed with 82% carbon (pulverized coke) havingthe following sieve analysis- Percentage Mesh 33 40-100 47 100-200 20Through 200 Example 52 Where using a plasticizer such as soapr stone inaddition to pulverized carbon and :mesh of the pulverized carbon can vaoil, I have found that the following composition produces satisfactorycastings Percentage Mesh 33 40-100 47 100-200 20 Through 200 In usingthe above compositions I have found that the castings produced present agood surface that is free from excessive oxidation, pitting and surfaceshrinks, and further I have found that the tensile properties, Brinellhardness, fatigue endurance and toughness of such castings are above theaverage of like articles cast in sand according to current practice.

In using my improved tion for casting, a small ortion of the volatileliquid binder is gasi ed bythe hot metal poured into the mold. After theso oured casting has cooled to a point where it will not ignite whenexposed to air, it can then be shaken out and the molding compositionreconditioned by adding thereto suflicient of the non-aqueous volatileliquid binder to moisten and place the composition in shape for reuse,it being noted that the other ingredients of the composition will besubstantially unaffected by casting condi tions. The volatile liquidbinder, preferably mineral oil, acts substantially the same in making myimproved molding composition green and moldable as does water in theconventional water bound green sand molds but without the bad effectsencountered due to water or steam giving up oxygen to oxidize the metal.While I have mentioned pul verized coke as the carbon material used inmy improved molding composition I do not wish to be limited to thatmaterial since I have found that pulverized coal and other similar hardhigh carbon compounds can be used in like manner. The term carbon willaccordingly be understood to broadly cover this class of materialsunless otherwise specified' Further, I have found that the considerablyfrom that specifically noted 1n the foregoing examples, the mesh useddepending upon the permeability and bond strength desired.

Other modes ofapplying the principle of my invention may be employedinstead of those explained, change being made as regards the method andproduct herein disclosed provided the method and product moldingcomposistated by any of the following claims or the dizable metal, whichconsists in forming a mold from a mixture of pulverized carbon and aplasticizer rendered moldable by being A mixed with a gasifiablenon-aqueous liqui metal consisting of t1c1zer and a gas] and castingsaid metal in said mold.

2. The process of casting a readily oxidizable metal, which consists informing a mold from a mixture of pulverized carbon and a plasticizerrendered moldable by being mixed with a gasifiable mineral oil, andcasting said metal in said mold.

3. The process of casting a readily oxidizablc metal, which consists informin a mold from a mixture of pulverized car on and pulverizedsoapstone rendered moldable by being mixed with a gasifiable non-aqueousliquid, and casting said metal in said mold.

4. The process of castin magnesium and alloys thereof, which consists informing a mold from a mixture of pulverized carbon and pulverizedsoapstone rendered moldable by being mixed with a p and casting saidmetal gasifiable mineral oil, in said mold. 5. A mold for casting.readily oxidizable ulverized carbon, a plasable non-aqueous liquidblnder.

6. A mold for casting magnesium and alloys thereof consisting pulverizedsoapstone and a medium body mineral oil binder.

' 7. A mold for casting magnesium and alloys thereof consisting ofapproximately 77 per cent of pulverized carbon, 10 per cent ofpulverized soapstone and 13 per cent of a medium body mineral oil.

Signed by me this 28th day of June, 1930.

' JOHN A. GANN.

of pulverized carbon,

